This invention relates generally to global positioning system satellite signal receivers, and more particularly to an overall architecture thereof and to specific improvements in radio frequency and digital processing receiver sections.
The United States government is in the process of placing into orbit a number of satellites as part of a global positioning system (GPS). Some of the satellites are already in place. A receiver of signals from several such satellites can determine very accurately parameters such position, velocity, and time. There are both military and commercial uses. A primary military use is for a receiver in an aircraft or ship to constantly determine the position and velocity of the plane or ship. An example commercial use includes accurate determination of the location of a fixed point or a distance between two fixed points, with a high degree of accuracy. Another example is the generation of a high accuracy timing reference.
In order to accomplish this, each satellite continually transmits two L-band signals. A receiver simultaneously detects the signals from several satellites and processes them to extract information from the signals in order to calculate the desired parameters such as position, velocity or time. The United States government has adopted standards for these satellite transmissions so that others may utilize the satellite signals by building receivers for specific purposes. The satellite transmission standards are discussed in many technical articles and are set forth in detail by an "Interface Control Document" of Rockwell International Corporation, entitled "Navstar GPS Space Segment/Navigation User Interfaces", dated Sept. 26, 1984, as revised Dec. 19, 1986.
Briefly, each satellite transmits an L1 signal on a 1575.42 MHz carrier, usually expressed as 1540f.sub.0, where f.sub.0 =1.023 MHz. A second L2 signal transmitted by each satellite has a carrier frequency of 1227.6 MHz, or 1200f.sub.0.
Each of these signals is modulate in the satellite by at least one pseudo-random signal function that is unique to that satellite. This results in developing a spread spectrum signal that resists the effects of radio frequency noise or intentional jamming. It also allows the L-band signals from a number of satellites to be individually identified and separated in a receiver. One such pseudo-random function is a precision code ("P-code") that modulates both of the L1 and L2 carriers in the satellite. The P-code has a 10.23 MHz clock rate and thus causes the L1 and L2 signals to have a 20.46 MHz bandwidth. The P-code is seven days in length. In addition, the L1 signal of each satellite is modulated by a second pseudo-random function, a unique clear acquisition code ("C/A-code") having a 1.023 MHz clock rate and repeating its pattern every one millisecond, thus containing 1023 bits. Further, the L1 carrier is also modulated by a 50 bit-per-second navigational data stream that provides certain information of satellite identification, status and the like.
In a receiver, signals corresponding to the known pseudo-random functions are generated and aligned in phase with those modulated onto the satellite signals in the process of demodulating those signals. The phase of the carriers from each satellite being tracked is measured from the results of correlating each satellite signal with a locally generated pseudo-random function. The relative phase of carrier signals from a number of satellites is a measurement that is used by a receiver to calculate the desired end quantities of distance, velocity, time, etc. Since the P-code functions are to be classified by the United States government so that they can be used for military purposes only, commercial users of the global positioning system must work only with the C/A-code pseudo-random function.
It is an object of the present invention to provide a global positioning receiver architecture that allows a reliable, low cost, low power consumption, simple receiver structure.
It is another object of the present invention to provide a receiver system that measures the relative phase of a number of satellite signals to a higher degree of accuracy than now possible, thus improving the accuracy of the ultimate quantities, such as position, velocity, and time, that are determined from those relative phase measurements.